Well ,the advantages of Xperia Mini Pro (yes it has keyboard ) far exceed 551
The only advantage of Galaxy 551 is light weight an .2 inches of screen ...
and it is easier to root ..
Advantages of Xperia Mini Pro .:
Snapdragon S2 V7 processor which meant Flash support and superior Game/app support Plus it has neon which means video playback is flawless
Faster UI than Stock Samaung .
Notification LED for messages and calls
Superior Dialer Options
Official ICS !!!!!
Good ROM/dev community (Not needed since performance wise Stock ROMs rival Custom ROMs) in case you want to go a step further
Superior screen in DayLight !!
Sound is lot better ,Stock Headsets are better.
Radio Reception is better (WIFi,Bluetooth,GPS ...)
One of the key points is Sony Customer Care/Forums are lot better .and the software bugs are minimal unlike Samsung
Also The Sony support staff are members of many Forums inclusive of XDA,AF and others ..
The list goes on and on .....
I may post on the blog about my experience
. In a day or two or today